Heat and pressure consolidated decorative laminates have been produced commercially for a number of years, and have found widespread acceptance as mar-resistant surfaces for wallcoverings, paneling, tabletops, countertops, and the like. These laminates contain a number of laminae that are consolidated to form a unitary structure carrying a surface decoration which can range from something as simple as a solid color to something as complex as an embossed simulated wood grain finish.
Although the methods of preparing such laminates and the number and types of laminae can vary widely, the procedure for preparing such laminates generally involves the consolidation of one or more sheets of core stock, depending primarily on the ultimate thickness desired, in combination with a decorative or print sheet and a top coat or overlay. The core stock usually comprises an unbleached kraft paper which has been impregnated with a relatively inexpensive thermosetting resin, such as a phenolic resin, which is easily cross-linked upon the application of pressure and heat. In order to prevent sticking of the laminate to the press platens specially treated caulstock or a release medium such as a paper or foil with a release coating surface are used between the platens and the laminate.
The decorative or print sheet has more stringent requirements than the core stock. It is usually pigment filled, must be capable of being impregnated with a noble thermosetting resin cross-linkable upon the application of heat and pressure, but exhibiting no color deterioration upon the application of such heat and pressure, and it must now allow any strike-through or bleeding of the resin used in the core stock. In many instances, the decorative or print sheet must also be capable of being printed with a design, such as simulated wood grain, which survives the consolidation step intact. Two of the most common noble thermosetting resins used to impregnate the decorative or print sheet are urea-formaldehyde resin condensates and melamine-formaldehyde resin condensates. Other resins such as polyester resins have also been so employed.
In most instances an overlay sheet is superimposed over the decorative or print sheet prior to consolidation of the laminae. The overlay sheet is generally a thin, high-quality, alphacellulose paper which is also impregnated with a noble thermosetting resin, usually the same type in the decorative or print sheet. The overlay sheet is usually transparentized during the consolidation step, thereby enabling the decoration and/or printing which is present on the decorative or print sheet to be readily seen in the finished laminate, yet imparting a greater degree of mar and abrasion resistance to the decorative laminate than would otherwise be obtained without such an overlay.
An alternative to using an overlay sheet is to employ a polymeric film between the decor sheet and the release medium. U.S. Pat. No. 3,616,021 discloses the use of a thermoplastic film in this manner. Films used in this manner are likely to be thermoplastic materials such as acrylics. Since the film is self-supporting it is necessarily thick and because of its rigid structure, the film is very brittle and handling of the film is difficult. Contamination of the film is also a problem since dust is picked up very easily by both sides of the film.
Another alternative to using an overlay sheet is to employ a film of noble thermosetting resin on a substrate having release properties. This approach provides compatibility between the resins in the film and the resin in the decorative sheet, but not a reaction between the resins of the film and the decorative sheet to produce a laminate with the improved surface properties of the invention.